The present invention relates generally to disc grinders for lignocellulosic material. More particularly, the present invention relates to refiner plate segments for such an apparatus.
In high consistency mechanical pulp refiners, the wood fibers are worked between two relatively rotating discs on which refiner plates are mounted. The plates usually have radial bars and grooves. A large volume of steam is produced between the plates as a result of this refining work. For effective refining, the fibers must be retained between the plates on the bar surfaces despite the high velocity of the flowing steam, and the enormous centrifugal forces. Typically, dams are provided in the grooves to interrupt material flow and thus improve the retention time of the material in the refining region.
In a typical refiner plate with radial bars and grooves, the bars provide impacts or pressure pulses which separate and fibrillate the fibers. The grooves enable feeding of the fibers and steam extraction. Near the perimeter of the plates, high radial steam flow and high centrifugal force both act to sweep the fibers outwardly from between the plates prematurely, thus reducing the refining effectiveness. The flow restrictions due to a small gap between the opposing plates and fiber-filled grooves result in a steam pressure peak between the plates, located radially inward from the perimeter. The force of this pressure peak on the discs is a major source of the refining thrust load, and can induce control instability at high motor loads.
It is thus desirable that the steam generated during refining be discharged from the refining region as quickly as possible, while the pulp is retained within the region as long as possible. The evacuation of steam is especially difficult for refiner plates which have a fine pattern of relatively narrow bars and grooves in the refining zone. Such plates produce heat from more fiber impacts per revolution than coarser refiner plates, and the grooves on fine refining zones are narrower, reducing the cross-sectional area available for the flow of steam. Consequently, the grooves in fine refining zones are prone to plugging due to the accumulation of the material being refined. Conventional refiner plates have a groove width greater than a certain minimum, to reduce the amount of plugging. Such a minimum groove width effectively limits the fineness of the refiner plate. In addition, the bars of fine refining zones are very narrow and have a height that is high in relationship to the width of the bar. Consequently, such bars are prone to breakage.